1. Field of the Invention
The present invention relates to a method of producing magnetic particles by a microreactor and a reaction method using a microreactor suitable for producing magnetic particles involving generation of by-product gas, and a microreactor.
2. Description of the Related Art
To increase magnetic recording density, it is important to make the particle size of magnetic particles contained in the magnetic layer small. For example, in the case of widely used magnetic recording media such as videotapes, computer tapes and diskettes, the smaller the noise, the smaller the particle size when the hard magnetic materials have the same mass.
As a potential material of magnetic particles for improving the magnetic recording density, CuAu or Cu3Au hard magnetic ordered alloy has been attracting attention (e.g., Japanese Patent Application Laid-open No. 2003-6830, Japanese Patent Application Laid-open No. 2001-256631). It is known that the hard magnetic ordered alloy has large magnetocrystalline anisotropy due to the strain generated when formed into an ordered alloy and the alloy has hard magnetic properties even if the particle size of the magnetic particles is made smaller.
Magnetic particles which have hard magnetic properties are prepared by a liquid phase method or a gas phase method, but the magnetic particles immediately after being prepared by a liquid phase method which is practical and excellent in mass production are random and have a face-centered cubic structure. Crystal having a face-centered cubic structure usually has soft magnetic properties or paramagnetic properties and not suitable for magnetic recording media. For magnetic recording media, it is necessary to obtain hard magnetic ordered alloy having a coercive force of not less than 95.5 kA/m (1200 Oe). To obtain such hard magnetic ordered alloy, it is necessary to transform the random phase to an ordered phase. For this reason, the production of magnetic particles is constituted by the step of preparing alloy particles capable of forming a CuAu or Cu3Au hard magnetic ordered alloy phase and the step of forming magnetic particles for forming CuAu or Cu3Au magnetic particles from alloy particles prepared in the step of preparing alloy particles, and usually annealing treatment (heating treatment) is carried out in the step of forming magnetic particles.
However, properties of magnetic particles used for magnetic recording media are not determined only by the step of forming magnetic particles, and the important point is how to produce, in the step of preparing alloy particles, minute alloy particles excellent in monodispersibility and excellent in ease in transformation which achieves easy transformation from a random phase to an ordered phase. The alloy particles are usually prepared by mixing a plurality of solutions for preparing alloy particles by the liquid phase method described above and as a mixing reactor, one having an agitation blade in a tank as shown in FIG. 8 has been suggested (Japanese Patent Application Laid-open No. 5-173267).
In the mixing reactor 1, to stir the solution filled in a tank 2, stirring blade 5 is rotated by rotational driving force from a motor 3 utilizing magnetic force via a communicating device 4 for non-contact communication. In addition, on the outer periphery of the tank 1, to control the temperature of the solution filled inside, a temperature control device 6 for heating or cooling the solution is provided. An introduction tube 8 for introducing a solution is provided on the sealing cover 7 of the tank 2 and exhaust tube 9 for exhausting the reaction mixture is provided at the bottom of the tank 2.
As seen in International Patent Application Japanese Laid-open Nos. 2001-521913 and 2001-521816, as a reactor for increasing the product yield and the purity of the reaction product and for conducting a reaction using dangerous or explosive reagent safely, microreactors for reacting a plurality of fluids using an extremely fine microchannel (also referred to as mixing channel) having a channel width of not more than 1 mm are drawing attention. Due to the above-mentioned micro scale of the microchannel in which a reaction is conducted, the flow of the fluid flowing through the microchannel is mainly in a laminar flow. This enables liquids to be reacted to flow through the microchannel in a laminar flow even without mechanical stirring and diffuse just by spontaneous movement of the molecules, whereby a prompt reaction is achieved. Accordingly, when magnetic particles are produced using such microreactor, minute alloy particles excellent in monodispersibility and excellent in ease in transformation can be produced.